1. Field of Inventions
The present inventions relate generally to medical devices that support therapeutic elements in contact with body tissue.
2. Description of the Related Art
There are many instances where therapeutic elements must be inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation and atrial flutter which lead to an unpleasant, irregular heart beat, called arrhythmia.
Normal sinus rhythm of the heart begins with the sinoatrial node (or “SA node”) generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular node (or “AV node”). This propagation causes the atria to contract in an organized way to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or “HIS” bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called “conduction blocks”) can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called “reentry circuits,” disrupt the normally uniform activation of the left and right atria. Because of a loss of atrioventricular synchrony, the people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis.
One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called “maze procedure” which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive. Maze-like procedures have also been developed utilizing catheters which can form lesions on the endocardium (the lesions being 1 to 15 cm in length and of varying shape) to effectively create a maze for electrical conduction in a predetermined path. The formation of these lesions by soft tissue coagulation (also referred to as “ablation”) can provide the same therapeutic benefits that the complex incision patterns that the surgical maze procedure presently provides, but without invasive, open heart surgery.
Catheters used to create lesions typically include a relatively long and relatively flexible body portion that has a soft tissue coagulation electrode on its distal end and/or a series of spaced tissue coagulation electrodes near the distal end. The portion of the catheter body portion that is inserted into the patient is typically from 23 to 55 inches in length, with another 8 to 15 inches, including a handle, outside the patient. The length and flexibility of the catheter body allow the catheter to be inserted into a main vein or artery (typically the femoral vein), directed into the interior of the heart, and then manipulated such that the coagulation electrode contacts the tissue that is to be ablated. Fluoroscopic imaging is used to provide the physician with a visual indication of the location of the catheter.
In some instances, the proximal end of the catheter body is connected to a handle that includes steering controls. Exemplary catheters of this type are disclosed in U.S. Pat. No. 5,582,609. In other instances, the catheter body is inserted into the patient through a sheath and the distal portion of the catheter is bent into a loop that extends outwardly from the sheath. This may be accomplished by pivotably securing the distal end of the catheter to the distal end of the sheath, as is illustrated in U.S. Pat. No. 6,071,279. The loop is formed as the catheter is pushed in the distal direction. The loop may also be formed by securing a pull wire to the distal end of the catheter that extends back through the sheath, as is illustrated in U.S. Pat. No. 6,048,329. One lesion that has proved difficult to form with conventional steerable and loop devices was the circumferential lesion that is formed within the pulmonary vein, or in the tissue surrounding the pulmonary vein, which isolates the pulmonary vein and cures ectopic atrial fibrillation.
More recently, catheters with inflatable energy emitting elements that are capable of forming circumferential therapeutic lesions have been proposed. Such energy emitting elements are solid, generally spherical, balloon-like structures that, when inflated, have a diameter which corresponds approximately to the diameter of the target tissue region. Examples of catheters with inflatable energy emitting elements are disclosed in U.S. Pat. No. 5,961,513. The inflatable elements are typically carried by a catheter and are deployed in a collapsed (or folded) and deflated state by way of a sheath whose distal end has been previously positioned near the target tissue region. After passing through the distal end of the sheath, the energy emitting elements are inflated and urged into contact with the target tissue. Energy is then transmitted to the tissue to form a lesion. Inflatable energy emitting elements are advantageous because their pliability facilitates superior tissue contact, which increases the likelihood that continuous lesions will be formed.
The present inventor has determined that conventional inflatable energy emitting elements are susceptible to improvement. For example, conventional inflatable elements occlude blood flow through the vein during use. Another issue identified by the present inventor is related to the fact that the circumference of a lesion formed by a conventional inflatable energy emitting element is dictated by the inflated circumference of the inflatable element. The formation of a lesion with a relatively large circumference requires an energy emitting element with a relatively large inflated circumference, which will also have a relatively large deflated circumference due to the amount of material required to produce the large inflated circumference. However, because inflatable energy emitting elements are advanced though sheaths, the deflated circumference must be smaller than the sheath lumens. The deflated circumference of a conventional energy emitting element is, therefore, the dimension that ultimately dictates the maximum lesion circumference. Similarly, conventional inflatable elements also often require the use of sheaths which are larger than would be otherwise desirable in order to accommodate a deflated inflatable element that will ultimately be inflated to the desired size.